Mast cells are multifunctional immune cells that play important roles in a number of allergic and inflammatory diseases including anaphylaxis and asthma. Activation of mast cells following cross-linking of high affinity IgE-receptors (Fc?RI) leads to the production of neuropeptides such as NP-FF, which are ligands for a novel G protein coupled receptor (GPCR) known as Mas-related Gene X2 (MrgX2). Of the human immune cells, MrgX2 is expressed ONLY in mast cells. A unique feature of MrgX2 is that it is activated by antimicrobial peptides (AMPs) such as the cathelicidin, LL-37, human ?-defensins and neuropeptides (NPs) substance P and NP-FF, which are produced by mast cells, lung epithelial cells and nerve endings. We therefore hypothesize that MrgX2 expressed in human mast cells contributes to the pathogenesis of anaphylaxis and asthma. However, MrgX2 is not expressed in mice and many of the peptides that induce degranulation in human mast cells via MrgX2 do not activate murine mast cells. To test our hypothesis, we will develop two in vivo mouse models that express human MrgX2 in mast cells. In aim #1, we will reconstitute Wsh/Wsh mast cell-deficient mice with murine bone marrow-derived mast cells (BMMCs) expressing human MrgX2 and a gain of function phosphorylation-deficient mutant ( ST-MrgX2). In aim #2, we will transplant human CD34+ hematopoietic stem cells into immune-deficient mice expressing growth factors for human mast cells to develop mice (humanized mice) that express MrgX2 in human mast cells in vivo. These two mouse models will be used to determine the role of MrgX2 on IgE-mediated cutaneous anaphylaxis and lung inflammation/airway hyperresponsiveness (AHR) in vivo. We believe that if the goals of this project are realized, it may provide a novel target for the treatment of allergic diseases that affect millions of Americans with billions of dollars in annual health care cost. Furthermore, development of these two model systems will provide new tools for future studies to determine the role of a human mast cell MrgX2 on other inflammatory and autoimmune diseases, such as rheumatoid arthritis, psoriasis, atherosclerosis, skin- blistering diseases, cancer and periodontal disease, in which mast cells are implicated.